The present invention relates to mounting structures for integrated circuit chips and more particularly to packages for mounting multichip modules on printed wiring boards.
It has become increasingly common to use multichip modules to mount subassemblies of integrated circuit chips on printed wiring boards. Such modules frequently include several integrated circuit chips supported on a planar silicon base which includes a fine-line multilayer structure for electrically interconnecting the chips. Most multichip modules are enclosed within packages which are intended to protect them from adverse environmental influences such as moisture and to allow for the modules to be mechanically attached and electrically connected to wiring boards. In the past, the packages for multichip modules have generally been metal or ceramic casings of the type used in thick and thin film hybrid microelectronics having small apertures within their side walls for allowing glass encapsulated leads to pass through the walls of the casings. The leads have served both to mechanically attach the package to the circuit board and electrically connect the module to the circuit board. The performance of such packages has been poor due to the delicate nature of this method of mechanical attachment and mismatches between the coefficients of thermal expansion of the components of the package and the circuit board on which the package is mounted.
Multichip module packages typically use a base to which four walls are applied, with a metal lid secured to the top of the sidewalls. The base and walls are usually made from the same material, and the lid may be of a different material: e.g., ceramic base and walls, and a KOVARTM.TM. lid. In these packages, close attention is paid to thermal expansion compatibility between the materials being used.
In many situations, it has become desirable to create alternative package configurations, wherein the sidewalls and the base may be made from different materials. An example of this might be a multichip package made from an aluminum nitride ceramic base and metal walls.
Matching of thermal expansion may be difficult; particularly over the wide temperature range typically used in brazing package parts together. In most cases, a modest thermal expansion mismatch can be accommodated over short distances. However, thermal expansion mismatch cannot usually be tolerated where longer distances are encountered, such as along one side of a package sidewall. When thermal expansion mismatch is possible, the fixation joint between metal sidewall elements and ceramic base will become mechanically stressed and subject to fracture or warpage.